1. Field of the Invention
The present invention relates generally to medical devices, and more particularly to endoluminal prostheses such as stents, or other implantable structures.
The prostheses may be placed in the arterial system, the venous system, or any other portion of the body. The use of stents may also be used to deliver drugs to tissue, support tissue, or maintain patency of body lumens, as well as performing other functions, and have been widely reported in the scientific and patent literature.
Stents are typically delivered via a catheter in an unexpanded configuration to a desired location in the body. The combined stent and catheter is typically referred to as the stent delivery system. Once at a desired location, the stent is expanded and implanted into the body lumen. Examples of locations in the body include, but are not limited to, arteries (e.g. aorta, coronary, carotid, cranial, iliac, femoral, etc.), veins (e.g. vena cava, jugular, iliac, femoral, hepatic, subclavian, brachiocephalic, azygous, cranial, etc.), as well as other locations including the esophagus, biliary duct, trachea, bronchials, duodenum, colon, and ureter.
Typically, a stent will have an unexpanded configuration with reduced diameter for delivery and an expanded configuration with expanded diameter after placement in the vessel, duct, or tract. Some stents are self-expanding, and some stents are mechanically expanded with a radial outward force applied from within the stent (e.g. with a balloon). Some stents have one or more characteristics common to both self-expanding and mechanically expandable stents.
Self-expanding stents are made from a material that is resiliently biased to return to a pre-set shape. These materials may include superelastic and shape memory materials that can expand to an implanted configuration upon delivery or through a change in temperature. Self-expanding stents are constructed from a wide variety of materials including nitinol (a nickel titanium alloy), spring steel, shape-memory polymers, etc.
In many stent delivery systems, particularly those used to deliver a self-expanding stent, the stent is typically retained on the catheter in its unexpanded form with a constraining member or other retention device such as a sheath or outer shaft. The stent may be deployed by retracting the outer shaft from over the stent. To prevent the stent from being drawn longitudinally with the retracting shaft, many delivery systems provide the catheter shaft with a pusher, bumper, hub, holder or other stopping element.
Precise delivery of stents can be challenging. In the case of balloon expandable stents, the stent may foreshorten as the stent radially expands, therefore, the change in length must be taken into account when deploying the stent at the treatment site. In the case of self-expanding stents, due to the elastic nature of the stents, they may “jump” away from the delivery catheter during deployment. Additionally, depending on the anatomy being treated, this may add further challenges to accurate stent delivery. In certain parts of the anatomy, longer stents may be needed to treat longer lesions or treatment regions. For example, with ilio-femoral and ilio-caval stenting, much longer stents are often required as compared with stenting of coronary lesions. This type of venous stenting may be used for the treatment of iliac vein compression syndrome (IVCS) and post-thrombotic syndrome (PTS) whereby the profunda and the inferior vena cava can be partially or completely blocked (or “stent jailed”) by the stent if the stent is not placed accurately after deployment. Because the stents are longer, they are often more difficult to load and onto a delivery catheter, and they may buckle during the loading process when a radial force is applied to the stent to reduce its diameter.
Additionally, deployment forces of radially strong or large diameter self expanding stents can be relatively high. Furthermore, deployment forces can be equally high with stents that are longer in length due to the added friction between stent and a constraining or protective sheath. These high deployment forces may cause the stent to axially or radially buckle when loaded or deployed because the longer stents are less supported and less rigid, they can also buckle during deployment. This is of particular concern when long self-expanding stents are used.
Providing a stent that avoids or has reduced potential for buckling during delivery allows the stent to overcome the excessive friction and avoid the bind up of the device during stent release. This is also desirable since incomplete or incorrect release of stent may require the user to remove the delivery system from the body at which time the stent may be unintentionally deployed in an undesirable location.
Therefore, it would be desirable to provide a stent used for treating longer lesions or longer treatment regions that has greater structural support and rigidity in order to resist buckling or unwanted deformation during loading onto a delivery system or during deployment in a patient.
At least some of these objectives will be met by the inventions described herein.
2. Description of the Background Art
Relevant patents and publications include U.S. Pat. Nos. 5,755,776; 6,261,318; 6,605,110; 6,749,629; 6,929,660; 7,122,049; 7,611,531; 7,722,661; and U.S. Patent Publication Nos. 2004/0204752; 2005/0116751; 2007/0055348; 2007/0255387; and 2009/0163989.